query: Add support for filters

This patch adds a “compiler” for query filters, converting them to a
callable function used while preparing the query result. In addition, a
hints call allows some analysis to be done on the query (e.g. referenced
names), making data collection more efficient.

The depth of filters is limited to avoid exceeding the runtime's maximum
recursion depth.

More operators and other improvements can be implemented using this
base. Extensive unittests are provided.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>